Typical physical networks contain several physical routers to perform L3 forwarding (i.e., routing). When a first machine wants to send a packet to a second machine located on a different IP subnet, the packet is sent to a router that uses a destination IP address of the packet to determine through which of its physical interfaces the packet should be sent. Larger networks will contain multiple routers, such that if one of the routers fails, the packets can be routed along a different path between the first machine and the second machine.
In virtualized networks, virtual machines on different subnets may need to communicate with each other as well. In this case, tenants may define a network for virtualization that includes both logical switches and logical routers. In these virtualized networks, techniques to minimize the effects of failover between logical router implementations are needed.